1. Field of the Invention
The present invention relates to a process for producing a solution of a high-molecular weight polycarbodiimide. More particularly, the present invention relates to a process for producing a solution of a high-molecular weight polycarbodiimide, having excellent storage stability by appropriately selecting the reaction conditions for said production, such as monomer types, monomer concentrations and the like.
2. Description of the Prior Art
It is well known to produce a polycarbodiimide by subjecting an organic diisocyanate to a decarboxylation and condensation reaction in the presence of a carbodiimidization catalyst. In this production, an aromatic diisocyanate has been used as the organic diisocyanate in most cases.
The applications of polycarbodiimides have been widened recently. As a result, it has been pointed out that the polycarbodiimides produced from an organic diisocyanate as mentioned above have a problem of generating aromatic amines having a bad odor, when subjected to treatments such as incineration and the like. Hence, polycarbodiimides derived from an aliphatic diisocyanate, free from the above problem have come to draw attention.
As one conventional process for producing a polycarbodiimide from an aliphatic diisocyanate, there is disclosed, in Japanese Patent Application Kokai (Laid-Open) No. 187029/1984, a process which comprises producing a polycarbodiimide solution from aliphatic mono-, di- and tri- isocyanates in an acetate type solvent (total monomer concentration=50% by weight or less) in the presence of a carbodiimidization catalyst (e.g. 3-methyl-1-phenyl-2-phospholene-1-oxide) at an appropriate temperature (120.degree.-160.degree. C.). This process, however, has had an inconvenience that nitrogen need be continuously sprayed in order to promote the completion of the reaction.
The present invention relates to a process for producing a polycarbodiimide derived from an aliphatic diisocyanate and has been completed as a result of extensive research made on the reaction conditions such as polymerization solvent, monomer concentrations, reaction temperature and the like. The present invention provides a process for producing a high-molecular weight polycarbodiimide solution of good stability very easily and simply, which process is free from the above-mentioned problems of the prior art.